1. Technical Field
The present invention relates to an electroluminescence (hereinafter, referred to simply as EL) element and a display device using the EL element.
2. Background Art
In recent years, among many kinds of flat-face-type display devices, high expectations have been drawn to display devices using electroluminescence elements. The display device using the El elements has such characteristics that it exerts a spontaneous light emitting property, is superior in visibility, has a wide viewing angle and is fast in response. Moreover, the currently developed EL elements include inorganic EL elements that use an inorganic material as a luminous body and organic EL elements that use an organic material as a luminous body.
In the inorganic EL element, for example, an inorganic phosphor such as zinc sulfide is used as a luminous body, and electrons accelerated by as high an electric field as 106 V/cm are caused to collide with the luminescence center of the phosphor so as to be excited, and light is emitted as those electrons are alleviated. Moreover, the inorganic EL elements include dispersion-type EL elements in which a phosphor layer formed by dispersing powdered phosphor in a polymer organic material or the like is prepared, with electrodes being formed on the upper and lower sides thereof, and thin-film-type EL elements in which two layers of dielectric layers are formed between a pair of electrodes, and a thin-film phosphor layer, sandwiched between the two dielectric layers, is formed. Among these, the former dispersion-type EL elements have low luminance with a short life, although they are easily manufactured, with the result that the application thereof has been limited. On the other hand, of the latter thin-film-type EL elements, those elements having a double insulating structure, proposed by Inokuchi, et al. in 1974, have been proven to have high luminance and a long life, and have been put into practical use as vehicle-use displays and the like, as described in Japanese Patent Publication No. S52-033491.
Referring to FIG. 13, the following description is made to discuss conventional inorganic EL elements. FIG. 13 is a cross-sectional view perpendicular to the light emitting face of a thin-film-type EL element 50 having the double insulating structure. This EL element 50 has a structure in which on a substrate 51, a transparent electrode 52, a first dielectric layer 53, a phosphor layer 54, a second dielectric layer 55, and a back electrode 56 are laminated in this order. An AC voltage is applied between the transparent electrode 52 and the back electrode 56 from an AC power supply 57 so that light emission is taken out from the transparent electrode 52 side. The dielectric layers 53 and 55 have a function of regulating an electric current flowing through the phosphor layer 54 so that they can prevent dielectric breakdown of the EL element 50 and function to provide a stable light-emitting characteristic. Moreover, a display device of a passive matrix driving system has been known in which transparent electrodes 52 and back electrodes 56 are patterned into stripes so as to be orthogonal to each other, and by applying a voltage to specific selected pixels in the matrix, a desired pattern displaying operation is carried out.
Dielectric materials to be used as the dielectric layers 53 and 54 preferably have a high permittivity, with high insulation resistance and high withstand voltage, and in general, dielectric materials having a perovskite structure, such as Y2O3, Ta2O5, Al2O3, Si3N4, BaTiO3, SrTiO3, PbTiO3, CaTiO3, and Sr(Zr, Ti)O3, are used. In general, inorganic phosphor materials to be used as the phosphor layer 54, on the other hand, have a structure in which an insulating material crystal is used as a host crystal doped with an element serving as a luminescence center. Since those materials that are stable physically as well as chemically are used as the host crystal, inorganic EL elements are superior in reliability and achieve a life for 30,000 hours or more. For example, the phosphor layer is mainly composed of ZnS and doped with a transition metal element or a rare-earth element, such as Mn, Cr, Tb, Eu, Tm, and Yb, so that the light emission luminance can be improved, as described in Japanese Patent Publication No. S54-8080).
In general, a compound semiconductor located between Group 12 to Group 16, such as ZnS used as the phosphor layer 54, has a polycrystalline structure. For this reason, there are many grain boundaries in the phosphor layer 54. Since these grain boundaries serve as diffusing bodies relative to electrons accelerated through electric field application, the exciting efficiency of the luminescence center is extremely lowered. Moreover, in the grain boundaries, a lattice strain becomes greater due to deviations and the like of the crystal orientation, and there are many non-irradiation recombination centers that give adverse effects on the EL light emission. Because of these influences, the light emission luminance of inorganic EL elements is low, failing to be practically used.
In order to solve the above-mentioned problems, methods for making greater the grain size of the crystal grain diameter of the phosphor layer and for improving the crystalline property thereof have been proposed. In accordance with the technique described in Japanese Patent Laid-open Publication No. H06-36876, an inorganic EL element is designed such that a first electrode has a specific crystal orientation, a first dielectric layer laminated thereon has a crystal orientation equivalent to that of the first electrode and a phosphor layer further laminated thereon has a crystal orientation equivalent to that of the first dielectric layer; thus, the grain boundary relative to the thickness direction is suppressed so that light emission luminance is improved. Moreover, in accordance with the technique described in Japanese Patent Laid-open Publication No. H06-196262, in the phosphor layer to which a rare-earth element has been added, by specifying the concentration of the rare earth element, the number of crystal growing cores in the initial growing period is set to a uniform and appropriate value. With this arrangement, pillar-shaped crystals having uniform particle sizes can be formed from the initial stage of the growth so that the light emission luminance can be improved.